Carbon nanotubes are used as electron sources due to their large aspect ratio, high conductivity, and the low field strengths needed to extract current. Among their uses is as field emission cathodes. Typical cathode arrangements involve films consisting of tangled, spaghetti-like carbon nanotubes or loose carpets of vertically aligned carbon nanotubes that have a typical density of 1-5% of their theoretical maximum density. Both configurations are derived from catalyst-induced growth of carbon nanotubes on a substrate. Although a single carbon nanotube can emit over 1 μA, obtaining high current densities from a large area carbon nanotube cathode is difficult to achieve. The problem with large area carbon nanotube cathode films comprising a plurality of individual carbon nanotubes is that the number of emission sites is limited by screening effects of the individual nanotubes and this in turn limits the current density of such large arrays. Currents also can be unstable under high field strengths. These limitations have prevented the realization of macroscopic carbon nanotube cathodes for high energy vacuum electronic applications. As a result, researchers continue to explore ways to use carpets and forests of single nanotubes as field emission cathodes for high energy applications. These initiatives include efforts to decrease the density of the single nanotubes to prevent or at least reduce their screening effects and thereby enhance the field concentration of these arrays of individual carbon nanotube emitters.
In certain vacuum electronic applications, electron beams of smaller diameter (<100 μm) and high current density (several mA) are required. For example, field emission DC cold cathodes find use for electron microscopy, novel x-ray sources, vacuum electronic devices, THz sources, and high power microwave tubes. Each of these applications typically requires high current densities with a high brightness electron beams driven by cathodes exhibiting long lifetime in the presence of deleterious conditions such as ion back bombardment and excessive heating. Small diameter (<50 μm) graphite fibers have been studied for these applications but they have not demonstrated the required robustness. They typically suffer serious degradation due to joule heating and produce at the most several hundred microamps before failure after tens of hours of operation. Similarly, carbon nanotube yarns that are spun from largely unaligned, multi-wall nanotubes are not durable enough for use as field emission cathodes due to their low density and the fact that they quickly degrade under high field testing.